If you play as a diver and switch from your SDAR to your pistol before entering the water, you aren't able to switch back to your SDAR as long as you are swimming and diving (http://feedback.arma3.com/view.php?id=11618).

Under those circumstances you can still change the fire mode of your SDAR (pressing F by default).

If you now press the key for your iron sight optic (right mouse button by default), you will get an "over the shoulder" view while swimming or a "hip heigh" view while floating (look at the attachment). {F20904} {F20905}